In page-lay.mkvi I see, at lines 1591–1594:

    \definepapersize
       [doubleoversized]
       [ \c!width=\dimexpr
    \paperheight+\layouttargetparameter\c!distance\relax,
        \c!height=\dimexpr2\paperwidth
    +\layouttargetparameter\c!distance\relax]


I would expect to see:

    \definepapersize
       [doubleoversized]
       [ \c!width=\dimexpr \paperwidth
    +\layouttargetparameter\c!distance\relax,
    \c!height=\dimexpr2\paperheight+\layouttargetparameter\c!distance\relax]


-- 
Rik